This invention relates to reactors for chemical reduction of nitrogen oxide (NOx) emissions in the exhaust gases of automotive engines, particularly diesel and other engines operating with lean air fuel mixtures that produce relatively high emission of NOx. More particularly, the invention pertains to single dielectric barrier planar and swept shaped non-thermal plasma reactors.
In recent years, non-thermal plasma generated in a packed bed reactor has been shown to be effective in reducing nitric oxides (NOx) produced by power plants and standby generators. These units usually have a reducing agent, such as urea, to enhance the conversion efficiency. The packed bed reactor consists essentially of a high voltage center electrode inserted into a cylinder of dielectric material, usually a form of glass or quartz. An outside or ground electrode is formed by a coating of metal in various forms, including tape, flame spray, mesh, etc. The space between the center electrode and the inside diameter of the dielectric tube is filled or packed with small diameter glass beads. When high voltage alternating current is applied to the center electrode, the surface of the beads go into corona, producing a highly reactive and selective surface for inducing the desired reaction in the gas.
Unfortunately, the packed bed design with its loose beads and glass dielectric is impractical for use in the conditions found in a mobile emitter, such as a car or truck. The vibration and wide temperature swings of the vehicle system would damage the packed bed and the necessary temperature and vibration isolation required to make it survive would not be cost effective.
Cylindrical or planar non-thermal plasma reactors are two common configurations for dielectric barrier discharge type reactors. Both of these configurations are characterized by the presence of one or more insulating layers in a current path between two metal electrodes, in addition to the discharge space.
Commonly assigned U.S. patent co-pending application Ser. No. 09/517,681 filed Mar. 2, 2000 entitled xe2x80x9cPlasma Reactor Design for Treating Auto Emissionsxe2x80x94Durable and Low Cost,xe2x80x9d which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element for conversion of exhaust gas constituents. The reactor comprises an element prepared from an extruded monolith of dense dielectric material having a plurality of channels separated by substantially planar dielectric barriers. Conductive material printed onto selected channels form conductive channels that are connected along bus paths to form an alternating sequence of polarity, separated by exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged at end portions of the monolith with a material suitable for excluding exhaust gases and preventing electrical charge leakage between conductive channels. Exhaust channels, disposed between opposite polarity conductive channels, are left uncoated and unplugged. During operation, exhaust gas flows through exhaust channels and is treated by the high voltage alternating current plasma field. The planar shape of the dielectric barriers provides a uniform electrical response throughout the exhaust channels.
Commonly assigned U.S. patent co-pending application Ser. No. 09/517,879 entitled xe2x80x9cPlasma Reactor For Treating Auto Emissionsxe2x80x94Durable And Low Costxe2x80x94Curved Shapes,xe2x80x9d which is hereby incorporated by reference herein in its entirety, and commonly assigned U.S. Pat. No. 6,354,903 entitled xe2x80x9cMethod Of Manufacture Of a Plasma Reactor With Curved Shape For Treating Auto Emissions,xe2x80x9d which is also incorporated by reference herein in its entirety, disclose a double dielectric barrier reactor and method for preparing same, respectively. The reactor is characterized by a reactor element prepared from a curved, swept-shaped substrate specifically designed for fabrication via extrusion. The as-extruded curved substrate comprises a thick outer wall surrounding a plurality of channels separated by dielectric barriers. Selected channels are coated with a conductive material to form conductor channels. The prepared reactor element comprises multiple concentric exhaust channels, multiple concentric conductor channels having alternating polarity, each connected to its respective polarity via bus paths, in-line structural support ligaments for providing optimal structural support while preventing exhaust leakage, and thick outer walls providing high crush resistance and allowing robust mounting into the reactor housing.
While the above non-thermal plasma reactors meet some of the current needs and objectives, they provide reactors having exhaust passages that are surrounded by dielectric material (xe2x80x9cdouble dielectric barrierxe2x80x9d reactors). There remains a need in the art for an improved, durable, low cost non-thermal plasma reactor and improved method of manufacturing same. There further remains a need for a non-thermal plasma reactor that can be prepared with reduced manufacturing complexity, reduced number of components and reduced overall material cost.
The present invention provides single dielectric barrier extruded substrate non-thermal plasma reactors having a plurality of single dielectric barriers defining exhaust channels between pairs of the single dielectric barriers. A first barrier layer of a pair of opposing dielectric barrier layers forming an exhaust channel therebetween is provided with a first polarity conductor on an interior surface thereof The second barrier layer of the pair of opposing barrier layers is provided with a second polarity conductor on the exhaust channel exterior surface, whereby the second barrier layer provides the single dielectric barrier layer. The second polarity conductor serves as a conductor on the channel interior surface of the adjacent exhaust channel. The layers are so provided in repetitive fashion until a reactor element of the desired size is achieved. In this way, the reactor provides a configuration that is free of dedicated conductive passages, providing increased active exhaust channel volume compared to comparable double dielectric barrier elements. This further provides improved overall volumetric efficiency.
The extruded reactor element includes cells defined by a single structural dielectric barrier comprising a xe2x80x9cconductor-single structural dielectric barrier-exhaust channel-conductorxe2x80x9d arrangement, wherein individual cells of the reactor element are defined by a single structural dielectric barrier rather than a double dielectric barrier.
The conductor may comprise conductive materials such as conductive coatings disposed on the single structural dielectric barrier, conductive plates or conductive tubes. In a preferred embodiment, conductors are connected to power and ground without the need for internal bus paths by extending conductive coating fully to the side walls. The bus paths provided on sides of the reactor provide further improved volumetric efficiency. When the reactor element is powered with high voltage alternating current, a non-thermal plasma is formed in the exhaust passages for treating constituents present in the exhaust stream passing through the exhaust passage, the present reactors being particularly suitable for mobile diesel applications.
Advantageously, the present single structural dielectric barrier NTP reactors provide lower manufacturing and material costs over NTP reactors employing double dielectric barrier designs.
The single structural dielectric barrier NTP reactors herein include extruded substrates having any desired shape, including, but not limited to, parallel plates, parallel plates having discrete spacers, C-shaped plates, I-shaped plates, and swept-shaped reactor substrates, such as round, oval, etc., shapes.
The present reactors provide an increase in volumetric efficiency of more than about 100% over comparable double dielectric barrier reactors The present NTP reactor and method comprises single dielectric barrier planar and swept-shaped reactor elements having minimal internal support ligaments. The planar and swept-shaped embodiments having a reduced number of ligaments alone or in combination with partially extending ligaments provide improved conversion efficiency and lower manufacturing cost over comparable designs having greater numbers of internal support ligaments.
In preferred embodiment, a swept-shaped reactor element, such as a concentric tube, is provided having an internal support ligament-free configuration. The ligament-free concentric tube embodiment provides increased conversion efficiency and thin wall capability over comparable single tube, reactors.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.